Add reply_chance to markov chains and markov commands

Every time a user sends a message, there is now a chance that markov
will reply with a random message based on what they said. !markov chance
allows lookups and setting of the chance that a user may receive a
message.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

lib/contrib/markov/chain.ex

@@ -3,7 +3,7 @@ defmodule Omnibot.Contrib.Markov.Chain do
   require Logger
 
   @enforce_keys [:order]
-  defstruct order: 2, chain: %{}
+  defstruct order: 2, chain: %{}, reply_chance: 0.01
 
   def train(chain, line) when is_binary(line) do
     train(chain, line |> String.split(~r/\s+/))